Thank you Per. I am really thankful for everyone's help, but to tell you the truth I thought Photoshop was for artists and web designers. And I am both.
Trust me. I get the whole save for web ideal. I have been using Photoshop for web design for fifteen years and have build many websites for clients. So I understand compression.
This is not about compression. These files are required to be large. It can be jpeg or png for Nuvango. Jpeg for Fine Art America. Patternbank.com requires at least 3500 px on one side.
I cannot save in larger sizes than 3000 px in CS 5.1.
Nuvango says:
"Firstly, it needs to be your artwork. Secondly, it needs to be big; ideally 6000px in either dimension (2000px minimum), up to 100MB in size, and saved in RGB format. Read our full upload guidelines. If you have any trouble, or have any questions at all, please just contact our friendly team for help."
Please bear in mind. They are not asking for .psd files. They want jpeg or png files.
6000 pixels will make the artwork look beautiful. Going under about 3500 pixels just defeats all my efforts to create fine artwork.
I really want to know if the suggestion made by the website photoshop.com is sound before I monkey around with my photoshop source files.
Photoshop: Save for Web & Devices can't save images that are 6000 x 3000px
Thanks everyone! All of your help is gratefully appreciated.